This invention relates to batteries.
Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material. In order to prevent direct reaction of the anode material and the cathode material, the anode and the cathode are electrically isolated from each other by a separator.
When a battery is used as an electrical energy source in a device, such as a cellular telephone, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
In a metal-air electrochemical cell, oxygen is supplied to the cathode from the atmospheric air external to the cell through one or more air access opening(s) in the cell container. The oxygen is an active material that is reduced to hydroxide ions at the cathode, which can include a catalyst supported on carbon particles. A metal, e.g., zinc, is another active material that is oxidized at the anode.
When used in devices, such as high power devices, it is desirable for the electrochemical cell to be able to support good current density.
The invention relates to batteries, such as metal-air batteries, that can sustain relatively high current density with relatively minimal voltage drop, for example, when used in high rate applications.
In one aspect, the invention features a cathode including carbon particles, for example, carbon blacks, that have been exposed to an oxidizing condition, for example, by contacting the carbon particles with an oxidizing agent such as an inorganic acid or an organic acid. Because the carbon particles have been oxidized, they have acidic surface functional groups, oxidized carbon functionalities, and/or a low pH, e.g., less than about 7, or between about 2 and about 4.
Without wishing to be bound to theory, it is believed that oxidized carbon particles have a chemically modified surface that is more polar than non-oxidized carbon particles. The oxidized carbon particles can draw more electrolyte into the cathode than non-oxidized carbon particles because the electrolyte can wet oxidized particles with polar surfaces more easily than the electrolyte can wet non-oxidized particles with relatively less polar surfaces. As a result, more of the cathode can be effectively used for the reduction-oxidation reactions that occur in the battery.
When incorporated in the cathode of a battery, for example, a metal-air battery, the oxidized carbon particles generally provide the battery with relatively high rate, which is useful for high power devices. The cathode has good physical integrity, with minimized cracks and/or holes. In embodiments in which the cathode has a dual-layer construction, the cathode has relatively low impedance, good adhesion, and good cohesion. While providing good current performance, the dual-layer cathode can also provide good protection against electrolyte permeation and leakage, e.g., when the cathode has a layer with oxidized carbon particles and a layer with non-oxidized carbon particles. Generally, cathodes with oxidized carbon particles are relatively simple and convenient to manufacture.
In another aspect, the invention features a method of making a battery having a cathode. The method includes contacting carbon particles with an inorganic acid or an organic acid, and incorporating the carbon particles into the cathode. The cathode can further include manganese dioxide and polytetrafluoroethylene. The cathode can be a monolayer cathode or a dual-layer cathode. The battery can be a metal-air battery, such as a zinc-air battery.
In another aspect, the invention features a cathode for a metal-air battery. The cathode may include a first layer having carbon particles that have not been treated with an acid, and a second layer having carbon particles that have been treated with an acid. The layers may include a catalyst and/or a polymer, such as polytetrafluoroethylene.
As used herein, xe2x80x9coxidized carbon particlesxe2x80x9d refer to carbon particles that have been exposed to an oxidizing condition, e.g., contact with an oxidizing agent. For example, oxidized carbon particles may have enhanced acidic surface oxides or functional groups, oxidized carbon functionalities, and/or a low pH. xe2x80x9cNon-oxidized carbon particlesxe2x80x9d refer to carbon particles that have not been exposed to an oxidizing condition.
Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.